Furniture with automatically movable members such as desks with an automatically height-adjustable table-top or cupboards with automatically movable doors are known. Here, “automatically movable” refers to a state where a possible movement of parts of furniture is not driven manually, but for example through a spring mechanism, a hydraulic or pneumatic mechanism or a motor-driven gear. Such a desk, as shown in FIG. 1, comprises a table-top 1, a supporting structure 2 and two table-legs 3. The table-legs are built in such a way that they can be moved telescopically and thereby are variable in length, and are connected to the supporting structure. The table-legs include, for example, an electric motor and a threaded rod for the automatic length-variation. By means of a control device which is not shown the electric motors are driven when an operator presses a switch (not shown), the threaded rod rotates, and the table-legs will be telescopically increased or decreased in length, depending on the rotation direction of the threaded rod.
The control device stops the electric motors when the operator either releases the switch, presses the same switch again or presses a further switch or also when a maximum or minimum position of the table-legs is reached. Since such a table-top is constructed to carry relatively heavy loads, such as several monitors or numerous books/files, the mechanic and the electric motors are built accordingly. This results in relatively high forces applied when the table-top is automatically height-adjusted. Further, other mechanisms may be used for the motion drive, such as a differently achieved electrically driven transmission or a hydraulic or pneumatic mechanism. Other devices, such as projectors, which can automatically be hidden in the table, monitors or the like may also act as movable members of furniture.
There are also other known furniture with automatically adjustable components, such as cupboards with movable doors as well as beds or bed-components, in particular, partly adjustable slatted frames.
Now using the example of a height-adjustable table-top: while adjusting, damage to the table or other objects, or even injuries of individuals who are above or below the table-top when it comes to a collision with the table-top and the object during height-adjustment may be caused. In order to avoid such risks, devices are applied for detecting collisions which record a collision of the table-top with an obstacle by use of one or more sensors and transmit information regarding this incident to the control device which then stops the operation of the electric motors.
EP 1 460 914 B1 discloses a device with adjustable elements, wherein there is provided a piezo-electric sensor, which in case of a detected collision, initiates a control signal which stops or reverses the movement.
Piezo-elements, however, are not ideal for the use in such objects. On the one hand, it is necessary to provide quite complex electronics, in order to enhance and then select the generated signals. On the other hand, the piezo-sensors disclosed in EP 1 460 914 B1 are not suitable for measuring a collision when initiating a movement, since an absolute measurement of the load can not be carried out. Due to the physical properties of the piezo-element, a charge displacement during the deformation is somewhat likely. This charge displacement causes a signal. Once the deformation is stopped, the signal also will be broken off. Due to the electronics connected, and the physical properties, electric charges still run off in this static state. This can result in incorrect measurements when measuring again, which can impair the reliability of the collision protection. At least, with this method, it is not possible to record permanent static loads.